An alternative theory of grasshopper life cycles

Sibly and Monk (1987) proposed a model of grasshopper life histories based upon the assumption that there is a trade-off between egg size and time taken to reach adulthood. They concluded that the length of the breeding period was the key environmental variable characterising seasonality. However, data on life history traits of Chorthippus brunneus from 28 field populations reared under standardised laboratory conditions fail to provide evidence for this trade-off. The key life history variation between populations of this species is the frequency with which an additional (IIa) instar is inserted during development. This additional instar permits individuals to reach a larger size at maturity.

At the growth and mortality rates reported for grasshoppers in the UK, length of season has relatively small consequences for fitness, so this is not the key environmental variable. The most important determinants of fitness are juvenile development time and mortality rate. In dull wet summers, when sunshine levels are low, juvenile development proceeds slowly. Fewer individuals reach maturity, so few eggs are laid leading to reduced numbers of animals in the next year.